DC7900 will not post with Intel Gigabit ET Quad Port Server Adapter

I have really struggled to get anywhere with this. My Intel Gigabit ET Quad Port Server Adapter, a PCI Express 4x add-in card, does not work in my DC7900 SFF (model FU042ET) with BIOS 786G1 v.01.26 (latest BIOS). The ME firmware is 5.2.50.1039 Rev. B.

This add-in card functions normally in another machine, and the DC7900 works normally without this card.

When I power on the DC7900 with the card installed, it gets as far as displaying a PXE boot initialization message, before hanging. The machine emits no beep codes (these are enabled), and displays no error message. The keyboard responds to setup keys, e.g. F10, because I see a message in the bottom right of the screen ("Setup"), but the Setup screen never appears. At that point, nothing else is possible, and I have to power down the machine.


Things I have tried:

I have tried the card in both the PCI Express slots that are 4x capable

I have swapped out the PSU for high-quality unit that has successfully powered the card.

I have disabled all on-board devices except video, unprovisioned AMT, and disabled Intel ME. (Tried all enabled and all disabled)

I have toggled the PCI SERR# generation (tried on and off)

I cleared the CMOS, and tried the default BIOS settings.

I have enabled and disabled all option ROMs on the Intel card (tried both)

I have used the Intel utility IOUTIL to both enable and disable I/O Map Mode on the add-in card (tried both).

I am thinking that the BIOS is not handling enumeration of this multi-function device, and maybe it has problems with the fact it contains a PCI Express bridge. However, these things are within the PCI and PCI Express specifications, and this card is a server-qualty device with what is regarded as a solid implementation by Intel.

I am thinking of scrapping the machine and trying an 8000 Elite instead, but am concerned because the BIOS for the 8000 seems superficially quite similar and also outdated, and the 8000 is based on the same chipset (Q45 Express).

Any ideas?